Osteosynthesis device

ABSTRACT

An osteosynthesis device having a bone screw, particularly a pedicle screw, a fork head that has a groove and a corrective pin seated in the groove of the fork head. A corrective pin is secured against twisting in that the bottom of the groove of the fork head is provided with a plurality of longitudinal slots lying parallel to one another.

FIELD OF THE INVENTION

The present invention relates to an osteosynthesis device having a bonescrew, particularly a pedicle screw, a fork head that has a groove and acorrective pin seated in the groove of the fork head.

BACKGROUND OF THE INVENTION

A large number of osteosynthesis devices of this type are known, forexample from German Utility Model DE-Gbm 91 04 027, European PatentApplication EP-A-346 521, German Patent Application DE-A 39 42 429,European Patent Application EP-A-443 894, European Patent ApplicationEP-A-348 272, European Patent Application EP-A-465 158, European PatentApplication EP-A-528 706, European Patent Application EP-A-443 892,German Patent Application DE-A 39 16 198, and German Patent PublicationDE-C 41 10 002. Various bone screws with forked heads are known fromthese references into which a corrective pin can be inserted andfixated. This primarily involves a non-slip bracing against the bonescrew. Only a slip-free and twist-free fixation of the corrective pinagainst the bone screw assures an optimum transfer of pulling andpressing forces onto the individual bones to be corrected and fixated,as well as the transfer of torque and bending moments.

As a rule, a good fixation is achieved in that a fastening screw isscrewed into the fork head in such a way that the screw presses on theinserted corrective pin. It has been shown, however, that an upwardbending of the fork head is not to be ruled out because of the highforces and moments occurring, so that the fastening of the correctivepin can loosen. Moreover, a clamping fixation of this type does notoffer adequate insurance against twisting of the corrective pin aroundits own axis.

With a clamping osteosynthesis implant (European Patent ApplicationEP-A-528 706), a fixing plate is placed onto the corrective pin insertedinto the fork head of the bone screw; the task of this plate on the onehand is to prevent the two legs of the fork head from being pressedapart and, on the other hand, it has longitudinal grooves on its sectionresting on the corrective pin that engage longitudinal grooves of thecorrective pin. By means of this, an increased insurance againsttwisting of the corrective pin in the fork head is achieved; however,the total torque acting on the corrective pin must be transferred ontothe bone screw via the pressing plate, a fastening screw and the twolegs of the fork head. At high torques, there is still the danger thatthe fork head will become deformed and, because of this, the fixation ofthe corrective pin will be loosened.

OBJECT AND SUMMARY OF THE INVENTION

Therefore an object of present invention is to further develop anosteosynthesis device of the type mentioned at the outset in such a waythat the corrective pin can be placed against the bone screw in a mannerthat is fixed against twisting.

This object is attained in accordance with the present invention in thatthe bottom of the groove of the fork head is provided with a pluralityof longitudinal slots lying parallel to one another.

The longitudinal slots provided at the bottom of the groove of the forkhead, that is, at the bottom of the groove into which the corrective pinis inserted, permit, on the one hand, a fixation secure against twistingof the corrective pin against the bone screw and, on the other hand, adirect transfer of force from the corrective pin onto the shaft of thebone screw. When a corrective pin with a smooth surface is placed intothe fork head, the longitudinal slots dig into the surface of thecorrective pin and fixate it in this manner. If the corrective pin isembodied as a threaded rod, then the longitudinal slots dig into thethread of the threaded rod. In an advantageous manner, the longitudinalslots extend in the direction of the axis of the corrective pin. Inother embodiments the longitudinal slots can also extend diagonally ortransversely.

The embodiment of the bone screw in accordance with the presentinvention has the advantage that the transfer of force from thecorrective pin onto the shaft of the bone screw is not effected viaholding elements, screws or the two legs of the fork head, but directlyfrom the pin onto the shaft. In this way significantly higher forces andmoments can be transferred, or the force or the moment can betransferred with greater insurance against slipping or slipping through.

In an advantageous manner, the corrective pin is provided withlongitudinal grooves, wherein the longitudinal grooves correspond to thelongitudinal slots and, in particular, extend in the direction of theaxis. In this embodiment the longitudinal grooves engage thelongitudinal slots of the fork head when the corrective pin is insertedinto the groove of the fork head, and produce a positive lockupconnection between the corrective pin and the bone screw. In thisinstance, both the corrective pin and the fork head can be made of ahard, deformation-resistant material. Hard or tough material isparticularly recommendable for the transfer of high forces and moments.

Problem-free insertion of the corrective pin into the groove of the forkhead and subsequent removal is achieved in that the longitudinal slotsare embodied without an undercut in the direction of the axis of thebone screw. Because the longitudinal slots do not have undercuts, thecorrective pin can be removed from the groove again without problemsafter a deformation during pressing into the longitudinal slots withoutbecoming caught in the longitudinal slots.

The longitudinal slots are preferably provided at the edge of the forkhead. This has the advantage that, because of the shorter longitudinalslots, substantially higher pressing forces can be attained duringpressing of the corrective pin, so that the longitudinal slots can digsignificantly better into the surface of the corrective pin if it issmooth. Moreover, by means of the higher pressure, the holding force isincreased with both smooth and grooved corrective pins.

The fork head of the bone screw is preferably provided with an outsidethread, particularly for a head nut. This head nut, which is screwedonto the corrective pin after the latter has been inserted into the forkhead, prevents the two legs of the fork head from bending apart under ahigh force effect. Moreover, by means of the head nut the corrective pincan be pressed into the bottom of the fork head.

It is provided in a further development that the head nut has an insidethread with a centering collar. This centering collar significantlyfacilitates the placement of the head nut onto the fork nut, so thatproblem-free, fast screwing on of the head nut is also possible indifficult-to-access positions, such as those that occur continuallyduring surgical operations, for example with ventral seating of thecorrective pin.

In an exemplary embodiment the head nut is embodied as a cap nut and hasa coaxial inside thread for a fastening screw. In this exemplaryembodiment the corrective pin is pressed into the bottom of the forkhead via the fastening screw.

In a further development it is provided that the fastening screw has acup point at the screwed-in end, and is particularly embodied as ahexagon socket screw. When the fastening screw is tightened, thescrewed-in end digs via the cup point into the area of the correctivepin located in the fork head, and the end additionally holds it againstslipping or twisting.

In an advantageous manner the axial front surface of the head nut, thatis, the top side of the head nut, is embodied to be ball-shaped,particularly spherical, and in particular is encircled by a hexagon. Viathe hexagon, the head nut can be screwed onto the fork head of the bonescrew and tightened in a simple manner. The spherical shape has theadvantage that the adjacent tissue experiences less irritation.

In a preferred exemplary embodiment, the bone screw has a screw bodythat extends conically from the point of the screw in the direction ofthe fork head. The conical shape of the screw body has the advantagethat the bone screw can be better fixated in the bone, and in particularsits tightly in the bones when screwed in. The bone screw is not heldonly via the threads, but also by means of the clamping effect of theconically expanding screw body.

The outer or nominal diameter of the bone screw, i.e., the threaddiameter, is preferably constant. In this case, in an advantageousmanner the width of the thread of the bone screw increases starting fromthe point of the screw. Because of this, a clamping effect is alsoobtained, by means of which the bone screw is additionally held againstthe bone. In the process, the thread pitch preferably remains constant.

In an advantageous manner, the end of the fork head connected to thescrew body is embodied to be ball-shaped, particularly spherical. Bymeans of the ball- or spherical shape, the advantage is attained that,even with an oblique or inclined arrangement of the bone screw on a boneplate, particularly in the distal region, it nevertheless rests fully inthe receiving cap of the bone plate.

Further advantages, features and details of the present invention ensuefrom the following description, in which a particularly preferredexemplary embodiment is represented in detail with reference to thedrawings. With the present invention, features shown in the drawings andmentioned in the description can be realized either individually in andof themselves, or in arbitrary combinations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of the bone screw in accordance with the presentinvention;

FIG. 2 is a side view in the direction of the arrow II of FIG. 1, partlyin section;

FIG. 3 is a top view of the bone screw;

FIG. 4 is a longitudinal section through a head nut;

FIG. 5 is a top view of the head nut;

FIG. 6 is an exploded representation of the upper part of the bone screwwith the corrective pin to be inserted, head nut and fastening screw;and

FIG. 7 is an assembly representation of the elements of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a bone screw, particularly a pedicle screw, indicated inits entirety by 1, that has a threaded shaft 2 and a fork head 3. Thethreaded shaft 2 comprises a screw body 4 with a screw point 5 and athread 6 on the outside of the screw body 4. It can be seen here thatthe screw body 4 extends conically from the screw point in the directionof the fork head 3. The diameter of the thread 6 remains constant,whereas the width of the thread 7 increases in the direction of the forkhead 3, starting from the screw point 5. However, the pitch of thethread 6 remains constant across the entire length of the thread shaft2. Moreover, it can be seen in FIG. 1 that the underside 8 of the forkhead 3 is embodied to be ball-shaped, particularly spherical, which isintended to be indicated by the line 9.

The fork head 3 has two legs 10 and 11, between which a groove 12 isdefined. The groove 12 has slightly conically-opening groove walls. Thebottom 13 of the groove 12 is provided with longitudinal slots 14, ascan also be seen in FIG. 3. Further, it can be seen that thelongitudinal slots 14 are cut without undercuts into the bottom 13 ofthe groove 12. As can be seen in FIGS. 1 through 3, the longitudinalslots 14 are located at the edge 15 of the groove 12, whereas thecentral region of the groove collar is free from longitudinal slots. Thelongitudinal slots 14 are formed either by means of milling or stamping.

The upper end of the fork head 3 is provided with an outside thread 17,which extends across approximately one-third of the height of the forkhead 3. A head nut 18 (FIG. 4) embodied as a cap nut can be screwed ontothis outer thread 17. At its lower end, the head nut 18 has a centeringcollar 19, which is followed by an inside thread 20 (first insidethread). Coaxial to the inside thread 20, the head nut 18 is providedwith a further inside thread 21 (second inside thread) that has asmaller nominal diameter. The top side of the head nut 18 is likewiseembodied to be ball-shaped or spherical. As can be seen from FIG. 5, thetop side of the head nut 18 has a hexagonal outside 22, by means ofwhich the head nut 18 can be screwed on the bone screw 1 and secured.

A fastening screw 23 (FIG. 6) can be screwed into the inside thread 21.The fastening screw 23 is likewise embodied on its top side to beball-shaped, and has a hexagon socket 24. On the underside, thefastening screw 23 is provided with a cup point 25 that presses onto acorrective pin 26 when the fastening screw 23 is tightened. Thiscorrective pin 26 (FIG. 6) is provided with longitudinal grooves 27,which correspond to the longitudinal slots 14 of the bottom 13 of thegroove 12.

When the corrective pin 26 is inserted into the groove 12 of the forkhead 3, as shown in FIG. 6, the longitudinal grooves 27 engage thelongitudinal slots 14 with positive lockup. Subsequently the head nut 18is screwed on, and the centering collar 19 significantly facilitates aplacement of the head nut 18 on the outside thread 17. Finally, thefastening screw 23 is screwed in and secured, wherein the cup point 25digs into the adjacent longitudinal grooves 27. In this way, thecorrective pin 26 is fixed against twisting by the longitudinal grooves27 that engage the longitudinal slots 14, and against displacement viathe cup point 25 dug into the longitudinal grooves 27. Moreover, torqueis transmitted by the corrective pin 26, via the positive lockupconnection with the longitudinal slots 14, directly into the screw body4 (FIG. 7).

We claim:
 1. An osteosynthesis device, comprising:a bone screw; and acorrective pin having an outer surface provided with longitudinalgrooves, wherein: said bone screw includes a fork head defining a grooveat the bottom of which a plurality of parallel, longitudinal slots areprovided, and an outside thread for receiving a head nut with an innerthread for engaging said outside thread; and said corrective pin beingseated in said groove of said fork head for engagement of said groovesof said corrective pin with said slots.
 2. The osteosynthesis device asdefined in claim 1, wherein said bone screw is embodied as a pediclescrew.
 3. The osteosynthesis device as defined in claim 1, wherein thelongitudinal extent of said slots is in the direction of the axisdefined by said corrective pin.
 4. The osteosynthesis device as definedin claim 1, wherein said longitudinal slots are embodied withoutundercuts in the direction of the axis defined by said bone screw. 5.The osteosynthesis device as defined in claim 1, wherein said groovedefines an edge, and wherein said longitudinal slots are provided atsaid edge.
 6. The osteosynthesis device as defined in claim 1, whereinsaid longitudinal grooves correspond to said longitudinal slots.
 7. Theosteosynthesis device as defined in claim 1, further comprising:a headnut defining an inner thread and a centering collar.
 8. Theosteosynthesis device as defined in claim 7, wherein said head nut isembodied as a cap nut defining a further inner thread which is coaxialwith said inner thread.
 9. The osteosynthesis device as defined in claim8, further comprising:a fastening screw, defining a cup point, saidfastening screw engaging said further inner thread.
 10. Theosteosynthesis device as defined in claim 9, wherein said fasteningscrew is embodied as a hexagonal socket screw.
 11. The osteosynthesisdevice as defined in claim 8, wherein said head nut defines an axialface end which is ball-shaped.
 12. The osteosynthesis device as definedin claim 11, wherein said axial face end is hexagonal.
 13. Theosteosynthesis device as defined in claim 1, wherein said bone screwincludes a body which defines a screw point at one end and said forkhead at its other end, and wherein said body extends conically from saidscrew point to said fork head.
 14. The osteosynthesis device as definedin claim 13, wherein the outside diameter of said bone screw isconstant.
 15. The osteosynthesis device as defined in claim 14, whereinsaid screw body widens to the diameter of said bone screw.
 16. Theosteosynthesis device as defined in claim 13, wherein said bone screwdefines a thread which increases from said screw point to said forkhead.
 17. The osteosynthesis device as defined in claim 16, wherein thepitch of said thread is constant.
 18. The osteosynthesis device asdefined in claim 1, wherein said fork head defines an axial end facewhich is ball-shaped.
 19. The osteosynthesis device as defined in claim18, wherein said ball-shape in spherical.
 20. A bone screw having:ascrew body defining a screw point at one end, and a fork head at theother end, said fork head having an outside thread for receiving a headnut, said screw body extending conically from said screw point in thedirection of said fork head; and a thread formed on said screw bodywhich extends from said screw point to said fork head, with the diameterof said thread being constant and the width of said thread increasingfrom said screw point to said fork head.
 21. The bone screw as definedin claim 20, wherein the pitch of said thread formed on said screw bodyis constant.